Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered to be comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, with the most proximal electrode serving as the anode and the most distal electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to induce a depolarization and a mechanical contraction of that chamber when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm and optimize the hemodynamics of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode. Dual chamber represents coordinated atrial and ventricular activity, with the atrial contraction occurring at the appropriate amount of time before the ventricular contraction.
Recently, there has been the introduction of pacing systems that stimulate in corresponding chambers of the heart as, for example, the right ventricle (RV) and left ventricle (LV). These are termed biventricular stimulation devices and may be programmed to operate in single, dual or tri chamber modes. Future designs may include the left atrium (LA) and operate in quad chamber modes.
Biventricular pacing has been shown to coordinate contractions of the left and right ventricles, reduce the amount of blood flow that leaks through the mitral valve, and decreases the motion of the septal wall that separates the chambers of the heart. Such motion can affect the quantity of blood that the ventricle can pump out in a single beat. Biventricular pacing has its greatest benefit when optimally timed after the atrial contraction, and the right ventricular and left ventricular contractions are also optimally timed.
Biventricular pacing has been found to be particularly advantageous in patient's suffering from congestive heart disease because of the improved ability of the left ventricle to fully pump blood from the heart. As a result, patients are able to tolerate greater exertion, have a longer life span, and experience a higher quality of life.
The objective of any pacing system is to reproduce the normal conduction timing patterns and mechanical contractions of cardiac tissue in patients whose conduction system and/or myocardial substrate is diseased and does not function within normal limits. It is known in the normal heart that the timing cycles between chambers of the heart become shorter as the heart rate becomes faster. The need for pacing systems to provide rate responsive timing as the normal heart would, which adjusts timing cycles according to the heart rate, and cardiac resynchronization therapy (CRT) has been shown.
The term “rate responsive” refers to the adjustment of various timing cycles in the device, based upon the heart rate. The heart rate may be the intrinsic sinus rate, the ventricular rate, or a pacemaker driven rate determined by the activity sensor. When the patient becomes more active, the sensor detects the motion, and if natural mechanisms do not increase the heart rate accordingly, the pacemaker will do so. Rate responsiveness has been extended to multiple parameters. These parameters include, for example, AV delay, post ventricular atrial refractory period, and other refractory periods.
In today's dual chamber atrio-ventricular pacing systems, AV delays may be programmed to shorten as rates become faster, reproducing the response of the normal, healthy heart. Biventricular pacing systems, provide cardiac resynchronization therapy (CRT) by pacing the LV and RV either simultaneously or with a fixed offset, synchronized with the atria in order to improve cardiac output and oxygen uptake. This offset, referred to as the interventricular pacing delay (IVPD), allows the clinician to program one of the two ventricles to be paced first, with the other to follow at programmable IVP Delay.
Based on the other rate responsive timing changes in both the healthy heart and their device counterparts in pacing systems, it may be beneficial for interchamber pacing delays to be adjusted accordingly in order to maintain expected cardiac output and oxygen uptake at different rates. The present invention addresses that need.